In inkjet printing, printing is conducted by spraying a liquid ink with a high degree of fluidity from very fine nozzles, and adhering that ink to a recording medium such as a sheet of paper. This type of inkjet printing enables the printing of high-resolution, high-quality images at high speed and with minimal noise, using a comparatively inexpensive printing apparatus, and is rapidly becoming widespread. Inkjet printing is conducted mainly using thermal systems that use heat to spray the ink, piezo systems that use application of a vibration to spray the ink, and electrostatic systems and the like.
In terms of the inks for these inkjet systems, aqueous type inks are becoming more prevalent, as they are capable of producing high-quality printed items at low cost. Aqueous inks are inks in which water has been included to enhance the drying properties, and compared with oil-based inks, offer the advantages of resistance to show-through (penetration of the ink through to the underside of the recording medium) even if double-sided printing is conducted onto plain paper, and favorable text reproducibility. The colorants used for these aqueous inks can be broadly classified into dyes and pigments, and pigment inks are rapidly gaining in popularity, as they offer superior levels of light resistance and water resistance.
Examples of problems that may occur when printing an aqueous ink onto paper such as plain paper include the phenomena of curling (convex curling) and cockling (wrinkling). These phenomena occur because the moisture within the aqueous ink causes cleavage of the hydrogen bonds between fibers within the paper, causing the paper to swell. If the paper deforms immediately following printing, then the printed sheets of paper may not align properly upon discharge from the printing apparatus, or if the degree of deformation is large, then the printed paper may become jammed inside the apparatus. These types of problems are more marked when the printing speed is high, such as for inkjet printers that employ a line head system. Furthermore, shortly after printing, the moisture that has penetrated into the paper begins to gradually evaporate, and this may cause deformation of the paper, and lead to problems in terms of handling and storage of the paper.
Furthermore, another known printing fault is the problem of satellites, which refers to small droplets that separate from the main ink droplets sprayed from the nozzles and are printed onto the paper as very fine specks. When air turbulence is caused inside the printing apparatus due to factors such as transporting of the paper, these small droplets can become suspended by the turbulent air flow, and may end up printed on the paper in the form of a wave-like pattern. This phenomenon is known as wave-like patterns.
Furthermore, aqueous inks also suffer from problems in terms of their performance when left to sit within an open system (hereafter referred to as “latency”). Namely, when an aqueous ink is left to sit in an open system exposed to the atmosphere, the water contained within the ink begins to evaporate, increasing the likelihood of aggregation of the solid matter such as pigments and the like within the ink, which may lead to solidification in the vicinity of the nozzles. If the nozzles become blocked with solid matter in this manner, then printing faults may occur.
Patent Document 1 discloses an inkjet aqueous ink, which contains a medium that exhibits specific levels of surface tension, viscosity and vapor pressure, and which exhibits improved recording head dischargeability and decap capabilities, as well as superior resistance to show-through, and excellent text quality and curling properties.    [Patent Document 1] Japanese Patent Laid-Open No. 2005-220296